18 REPORT— 1879. 



animal kingdom can in general be most satisfactorily observed, and the 

 greater number of recent researches into the nature of these phenomena 

 have found their most fertile field in the early periods of the development 

 of the egg. 



A discussion of the still earlier changes which the egg undergoes in 

 order to bring it into the condition in which cell-multiplication may be 

 possible, would, however full of interest, be here out of place ; and I shall 

 therefore confine myself to the first moments of actaal development — to 

 what is called ' the cleavage of the egg ' — which is nothing more than a 

 multiplication of the egg cell by repeated division. I shall farther confine 

 myself to an account of this phenomenon as presented in typical cases, 

 leaving out of consideration certain modifications which would only 

 complicate and obscure our picture. 



The egg, notwithstanding the preliminary changes to which I have 

 alluded, is still, at the commencement of development, a true cell. It 

 has its protoplasm and its nucleus, and it is, as a rule, enveloped in a 

 delicate membrane. The protoplasm forms what is known as the vitellus, 

 or yolk, and the surrounding membrane is called the ' vitellary mem- 

 brane.' The division which is now about to take place in it is introduced 

 by a change of form in the nucleus. This becomes elongated, and assumes 

 the shape of a spindle, similar to what we have already seen in the cell- 

 division of plants. On each pole of the spindle transparent protoplasm 

 collects, forming here a clear spherical area. 



At this time a very striking and characteristic phenomenon is 

 witnessed in the egg. Bach pole of the spindle has become the centre of 

 a system of rays which stream out in all directions into the surrounding 

 protoplasm. The protoplasm thus shows, enveloped in its mass, two 

 sun-like figures, whose centres are connected to one another by the 

 spindle-shaped nucleus. To this, with the sun-like rays streaming from 

 its poles, Auerbach gives the name of ' Karyolitic figure,' suggested by 

 its connection with the breaking up of the original nucleus, to which our 

 attention must next be directed. 



A phenomenon similar to one we have already seen in cell-division 

 among plants now shows itself. The nucleus becomes broken up into 

 a number of filaments, which lie together in a bundle, each filament 

 stretching from pole to pole of the spindle. Exactly in its central point 

 every filament shows a knot-like enlargement, and from the close approxi- 

 mation of the knots there results a thick zone of protoplasm in the 

 equator of the spindle. Each knot soon divides into two halves, and 

 t.'ach half recedes from the equator and travels along the filament towards 

 its extremity. When arrived at the poles of the spindle each set of half- 

 knots becomes fused together into a globular body, while the intervening 

 portion of the spindle, becoming torn up, and gradually drawn into the 

 substance of the two globular masses, finally disappears. And now, 

 instead of the single fusiform nucleus whose changes we have been 

 tracing, we have two new globular nuclei, each occupying the place of 



